With recent spread of the Internet, services for handling a large volume of content, such as video, have been rapidly expanded. As a result, a capacity of an optical communication system, which is a backbone network, has also been increasing. In such a large capacity communication system, it is important to efficiently utilize limited optical spectral regions.
In a wavelength division multiplexing (WDM) technique used in an optical communication system, one carrier (carrier wave) is generally disposed in each channel in certain center frequency intervals. Such a configuration of channels is defined by a frequency grid. A frequency grid with the center frequency interval of channels of 50 GHz, for example, is referred to as a “50 GHz grid” in the following. Further, as for channels constituting a WDM signal, “a center frequency of a channel” and “a center frequency interval of channels” are hereinafter simply referred to as “a center frequency” and “a center frequency interval”, respectively, unless otherwise specified.
Meanwhile, in order to increase a capacity of a transmission system, there is a demand for improving an accommodation efficiency of a WDM signal by changing the frequency grid to a frequency grid having an optimum center frequency interval in accordance with a transmission rate or a modulation method. For example, there is a demand for transmission of a WDM signal in which frequency grids having different center frequency intervals, such as a 50 GHz grid, a 37.5 GHz grid, and a 25 GHz grid, coexist.
In view of the above-mentioned circumstances, in ITU-T Recommendation G.694.1 defining frequency grids, in addition to a fixed grid, a flexible grid was standardized in 2012. ITU-T stands for the International Telecommunication Union Telecommunication Standardization Sector. In the flexible grid, a center wavelength of 193.1+n×0.00625[THz] (n is an integer) and a slot width of 12.5×m[GHz] (m is a positive integer) are defined.
In connection with the present invention, PTL 1 describes an optical transmission apparatus including a channel monitor that outputs information about an operation channel. PTL 2 describes a signal light monitoring apparatus that receives split monitor light and determines whether or not signal light is present for each channel to be measured.